This invention relates to fuel carburetion, and in particular to an improved carburetor having a fuel metering system for supplying a homogeneous mixture of fuel and air across the throttle opening of the carburetor while precisely controlling fuel and air flow rates.
For so long as internal combustion engines have been in existence, various carburetors have been developed to supply a required air-fuel mixture to the engine to promote proper and efficient combustion. Although myriads of carburetion schemes and devices have been developed, a continuing problem has been metering of the air-fuel mixture in a consistently homogeneous blend such that the air-fuel mixture received by each cylinder of the internal combustion engine is essentially the same as that supplied to each other cylinder.
In addition, not only is it important to control the homogeneity of the fuel-air mixture, it is also important to control the actual quantity of the fuel injected into the air stream in relation to the density of the air passing through the carburetor. Thus, when the air density decreases, it is important to also reduce the fuel flow rate so that the air-fuel blend supplied to the internal combustion engine is not fuel rich. This is particularly important in aircraft, where at high altitudes, the air density is considerably reduced. A commensurate reduction in the flow rate of the fuel must be made in order to properly lean the mixture to avoid fuel waste or possible engine flooding.
In conventional carburetors or fuel injection systems, the velocity of the air passing through a venturi portion assumed to correspond directly to the air mass flow. This assumption remains correct so long as there is no change in air density. If the ambient air temperature or pressure does change, then the resultant change in density invalidates this assumption and the carburetor or injection system experiences a change in air-fuel ratio. If the air density increases, then the air-fuel ratio becomes leaner and if the air density decreases, then the air-fuel ratio becomes richer. In most carburetor applications except aircraft the recent low cost of fuel has made mixture control not cost effective. In aircraft, where density-related mixture changes due to altitude result in large power reductions mixture control has always been a necessary feature.
The venturi system of measuring air flow and metering fuel is based upon the Bernoulli principle as expressed by the Bernoulli equation as follows: EQU 1/2V.sup.2 +P/.rho.=Constant
where P=Pressure, V=airflow velocity, .rho.=air density. As the Bernoulli equation applies to air flow in a venturi, it can be rewitten as follows: EQU 1/2(V.sub.2.sup.2 -V.sub.1.sup.2)+[(P.sub.2 -P.sub.1)/.rho.]=0
or, EQU 1/2(V.sub.2.sup.2 -V.sub.1.sup.2)=[(P.sub.1 -P.sub.2)/.rho.]
with the subscripts 1 and 2 referring to different axial locations in the flow tube. If the velocity at location 2 is high (such as occurs at the throat of a venturi) the pressure is lower than the pressure at a location where the velocity is low. From the Bernoulli equation, it is seen that the amount of pressure difference is much greater than the velocity difference because the velocities in the equation are squared.
The pressure that is sensed in a direction perpendicular to the direction of local flow in a venturi is the static pressure and is equal to that which would be sensed by a pressure instrument moving with the air flow. The pressure that is sensed by a probe inserted in the flow path and oriented with its opening facing the oncoming air is defined as the total pressure. The difference between the total pressure and the static pressure is the dynamic pressure and is related to the flow velocity by Bernoulli'equation as follows: EQU P.sub.T -P.sub.S =1/2.rho.V.sup.2
In the absence of friction, the total pressure remains constant along the length of a flow tube or venturi. In an area where the flow velocity increases due to a constriction in flow area, the static pressure is commensurately low.
Slide-type carburetors consisting of an air passage and a throttle plate movable to provide an adjustable throttle opening to alterably constrict the air passageway have been in existence for some time, as evidenced by U.S. Pat. Nos. 3,709,469 and 3,957,930. Such devices provide for throttling of the air flow in combination with mechanical control of the fuel quantities added to the carburetor. However, because fuel is injected into one side of the throttle opening in either of these devices, they suffer from an inability to supply a homogeneous air-fuel mixture across the throttle opening and do not permit a fuel range of air-fuel mixture control.
Other devices are known for metering fuel flow across the throat of a carburetor, as evidenced by U.S. Pat. Nos. 1,142,763 and 4,205,024. While such devices do permit fuel distribution effectively across the carburetor, it is difficult with such devices to adjust the air fuel mixture as the carburetor air passageway is throttled.